1. Field of the Invention
The present invention relates to radio communication systems implementing radio communications using wireless terminals via base stations by way of radio media and in particular to communication characteristic measuring devices that measure communication characteristics so as to switch over radio media in wireless terminals.
The present application claims priority on Japanese Patent Application No. 2008-329901, the content of which is incorporated herein by reference.
2. Description of the Related Art
Recently, various radio media such as wireless local area networks (LAN) and cellular phone networks have been developed, and various technologies have been developed to seamlessly integrate a variety of radio media.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-85759    Non-Patent Document 1: Tadayuki Fukuhara, Sachiko Yamanaka, Akira Yamaguchi, Sadao Obana, “Delay measurement method for cognitive wireless access network”, the Institute of Electronics, Information and Communication Engineers, 2006 Society Convention, B-17-19, September 2006    Non-Patent Document 2: 3GPP TS36.321 “E-UTRA MAC Protocol Specification”    Non-Patent Document 3: 3GPP TS36.322 “E-UTRA MAC Protocol Specification”    Non-Patent Document 4: Contribution to 3GPP, 3GPP TSG RAN WG2 #62 R2-082229 “HARQ Process Management for Persistent Scheduling”
Patent Document 1 discloses that a variety of radio communication systems is integrated using cognitive terminals and cognitive base stations by way of a variety of radio media. Cognitive base stations detect radio environments so as to select radio media based on cognition results.
Non-Patent Document 1 discloses that delay time differences occurring between communication systems are measured as one indicator for detecting radio environments. Specifically, gateways (which are higher places in the hierarchical structure of communications than base stations) transmit “probe packets” exclusively used for measurement to terminals via various communication systems so as to measure delay time differences occurring between communication systems.
Non-Patent Document 2 discloses the standard for Media Access Control (MAC) of “Long Term Evolution (LTE)” in 3.9-Generation mobile system, which includes Hybrid Automatic Repeat Request (HARQ) for repeating packets. According to the HARQ in connection to uplink communication (for transmitting packets from terminals to base stations) and downlink communication (for transmitting packets from base stations to terminals), packet receivers (i.e. base stations in uplink communication or terminals in downlink communication) determine success or failure of reception every time they receive packets, wherein they send back “acknowledgement” (ACK) in the case of success of reception but sends back “negative acknowledgement” (NACK) in the case of failure of reception; hence, packet transmitters (i.e. terminals in uplink communication or base stations in downlink communication) repeat transmitting packets until they receive ACK.
Non-Patent Document 3 discloses the standard for the LTE regarding the repetition (or retransmission) of packets. The ARQ is a repetition method in a higher place of communication hierarchy than the HARQ. When packets are repeated the maximum number of times according to the HARQ but fail to reach destinations, the ARQ is activated to repeat packets again. The ARQ is mostly used as a remedy against feedback errors occurring in the HARQ.
Non-Patent Document 4 discloses the operation of the HARQ adapted to Voice over Internet Protocol (VoIP) communication, which normally employs “Persistent Scheduling” for repeating packets in units of 20 milli-seconds but which employs “Dynamic Scheduling” for dynamically determining the time for repeating packets in accordance with the HARQ. It discloses that the period of time required for repeating packets upon one HARQ is set to 8 milli-seconds, for example.
These conventional technologies have various drawbacks. That is, Patent Document 1 does not disclose the specific configuration for detecting radio environments. Non-Patent Document 1 suffers from the following drawbacks.    (a) The delay time characteristics are not determined per each individual communication system.    (b) Probe packets exclusively used for measurement consume communication resources for the purpose of measurement.    (c) Gateways are required to transmit probe packets and may easily face the shortage of processing abilities to cope with the increased number of terminals.
In real-time applications such as VoIP, it is important to detect delay time variations rather than delay time lengths. Pertaining to the LTE, Non-Patent Document 2 discloses that the ACK (representing the success of reception) or NACK (representing the failure of reception) is sent back per each packet, while Non-Patent Document 4 discloses that packets are repeated per predetermined period of time. Considering the above characteristics, it may be necessary to monitor the reception acknowledgement information such as ACK and NACK, and it may be necessary for terminals to measure communication characteristics representing delay time variations in uplink communication and downlink communication independently.